After Upgrade to Mojave cannot AirPlay to Apple Express!!

I keep all my music on an iMac (21.5-inch, Late 2013) which I recently upgraded to macOS Mojave (10.14.6).

I have several Apple AirPort Express routers (2nd Generation - running the latest Firmware 7.8.1) which I use to distribute my music. The Express routers are all hard-wired to my network as is the iMac. This has worked seamlessly for quite a while.

Following the recent upgrade to Mojave, on my iMac, when I try to stream the music residing on the iMac to one of the Express routers (AirPlay) from either iTunes or using Apple Remote app from my iPhone (which is effectively the same since remote is controlling the music from the iMac) I get an error! iTunes (and Remote) sees the AirPort Express routers but cannot connect to them!

Generally the error says that I cannot connect to the device (the Express). However, I have seen other errors including "Airport Express requires an Apple Music membership to play this"!! I don't understand this at all!

When I try to stream to the Express directly from iTunes (as opposed to the Remote app) I see that after I select the Express router as Airplay output, it displays a message "Express Updating"... not sure what this refers to!!

Interestingly, after all the problems connecting using Remote or directly with iTunes, I decided to use my iPhone stream music to the same Express using Airplay to play music on my iPhone. This worked perfectly as it should.

Then after playing the music from my iPhone on the Express for a while I try to connect to the same Express from iTunes and then it seems to connect but not play the music until I kill the music streaming on my iPhone. Then after I do kill the music on my iPhone I the streaming from iTunes works!!

This is weird and a real pain if I have to do this every time I want to stream music! Any thoughts on how I can fix this?